<p>Overly complicated regular expressions are hard to read and to maintain and can easily cause hard-to-find bugs. If a regex is too complicated, you
should consider replacing it or parts of it with regular code or splitting it apart into multiple patterns at least.</p>
<p>The complexity of a regular expression is determined as follows:</p>
<p>Each of the following operators increases the complexity by an amount equal to the current nesting level and also increases the current nesting
level by one for its arguments:</p>
<ul>
  <li> <code>|</code> - when multiple <code>|</code> operators are used together, the subsequent ones only increase the complexity by 1 </li>
  <li> <code>&amp;&amp;</code> (inside character classes) - when multiple <code>&amp;&amp;</code> operators are used together, the subsequent ones
  only increase the complexity by 1 </li>
  <li> Quantifiers (<code>*</code>, <code>+</code>, <code>?</code>, <code>{n,m}</code>, <code>{n,}</code> or <code>{n}</code>) </li>
  <li> Non-capturing groups that set flags (such as <code>(?i:some_pattern)</code> or <code>(?i)some_pattern</code>) </li>
  <li> Lookahead and lookbehind assertions </li>
</ul>
<p>Additionally, each use of the following features increase the complexity by 1 regardless of nesting:</p>
<ul>
  <li> character classes </li>
  <li> back references </li>
</ul>
<p>If a regular expression is split among multiple variables, the complexity is calculated for each variable individually, not for the whole regular
expression. If a regular expression is split over multiple lines, each line is treated individually if it is accompanied by a comment (either a Java
comment or a comment within the regular expression), otherwise the regular expression is analyzed as a whole.</p>
<h2>Noncompliant Code Example</h2>
<pre>
if (dateString.matches("^(?:(?:31(\\/|-|\\.)(?:0?[13578]|1[02]))\\1|(?:(?:29|30)(\\/|-|\\.)(?:0?[13-9]|1[0-2])\\2))(?:(?:1[6-9]|[2-9]\\d)?\\d{2})$|^(?:29(\\/|-|\\.)0?2\\3(?:(?:(?:1[6-9]|[2-9]\\d)?(?:0[48]|[2468][048]|[13579][26])|(?:(?:16|[2468][048]|[3579][26])00))))$|^(?:0?[1-9]|1\\d|2[0-8])(\\/|-|\\.)(?:(?:0?[1-9])|(?:1[0-2]))\\4(?:(?:1[6-9]|[2-9]\\d)?\\d{2})$")) {
    handleDate(dateString);
}
</pre>
<h2>Compliant Solution</h2>
<pre>
    if (dateString.matches("^\\d{1,2}([-/.])\\d{1,2}\\1\\d{1,4}$")) {
        String dateParts[] = dateString.split("[-/.]");
        int day = Integer.parseInt(dateParts[0]);
        int month = Integer.parseInt(dateParts[1]);
        int year = Integer.parseInt(dateParts[2]);
        // Put logic to validate and process the date based on its integer parts here
    }
</pre>
<h2>Exceptions</h2>
<p>Regular expressions are only analyzed if all parts of the regular expression are either string literals, effectively final local variables or
<code>static final</code> fields, all of which can be combined using the '<code>+</code>' operator.</p>
<p>When a regular expression is split among multiple variables or commented lines, each part is only analyzed if it is syntactically valid by
itself.</p>

